The photo below was taken about 5 years after the start of the Blackbird's test program and I'm unaware of any revolutionary advances in tracking or photography that occurred in the interim. The subject vehicle is somewhat larger but is moving faster (6,164 mph) and at higher altitude (220,000 ft.) than the Blackbird family is generally believed capable of.
1) It's not just somewhat larger, it's a *lot* larger.
2) The vehicle isn't approaching from the dim vague distance, but was clearly observed from the beginning, RIGHT OVER THERE, with massive camera tracking rigs built for the purpose.
3) Launch vehicles go up and over, not more or less purely horizontal. That Saturn V isn't visible low in the sky, but *high* in the sky. Far less air to look through than something lower-yet-closer but closer tot he horizon.
4) Launch vehicle trajectories are pre-programmed; but whenever there's a divergence the manual trackers tended to lose them almost instantly.
5) Launch vehicles were painted *white* in large part so that they could be clearly seen. Look at the bottom of the Saturn V S-IC stage in your photo: it's dark... and essentially the same color as the background. All that you can see of the Saturn V is what's painted white; the black bits are truly invisible.
In order to be seen for a 1965 speed record run, a YF-12A was painted with a white cross. Without it, it would have been damned difficult to see at altitude.
Before assuming some whackaloon conspiracy about the SR-71 family using "plasma stealth" or some such rubbish as an excuse for why it hasn't been photographed... find some photos of the U-2/TR-1 at altitude. Roughly the same color as the SR-17, roughly the same size (TR-1 span 103 feet, SR-71 length 107 feet), but flies a little lower (70K vs 85K) and a *lot* slower. Should be vastly easier to spot and photograph. Go ahead, look 'em up. Should be oodles of them.
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